Converts the value of input UV to polar coordinates. In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction.
The resulting effect is that the x channel of the input to UV is converted to a distance value from the point specified by the value of input Center and the y channel of same input is converted to the value of an angle of rotation around that point.
These values can be scaled by the values of inputs Radial Scale and Length Scale respectively.
From the node wiki.